Patent Application
20080069952
The cleaning method described herein, in an exemplary embodiment, involves a hydrogen-reduction process for removal of native oxide, organic contaminants, and metal contaminants from a surface of a silicon wafer. Depending upon a cleanliness level of incoming starting materials (e.g., silicon wafers or other substrates), a standard RCA-type clean may be used for removal of, for example, particulate contaminants and other gross impurities prior to the hydrogen-reduction process. The hydrogen-reduction process may therefore, in particular applications, be combined with a standard wet-cleaning process.
The method is partially based on a chemical reaction between the silicon dioxide and the hydrogen, thus taking advantage of a reducing effect of hydrogen. For example, when a native oxide film, SiOx is exposed to hydrogen, H2, inside a process chamber, the chemical reaction breaks the native oxide down into silane, SiH4, and water, H2O, such that
SiOx+ SiH4+H2O+H2
(Note that native oxide frequently contains dangling bonds such that an SiO2 composition may be only partially formed. Thus, the reaction shown is not fully balanced.)
With reference to
Although the source gases are shown as pure hydrogen and oxygen, one skilled in the art will recognize that other precursor gases that are hydrogen-containing or oxygen-containing may be used and properly fractionated or dissociated as needed. Additionally, any appropriate oxidant may be utilized in place of the oxygen source such as, for example, steam. Also, pure O2 or steam may be diluted with an inert gas, such as nitrogen. Alternatively, a percentage of H2 (approximately 1% to 33% by volume) injected into an oxygen ambient in a reduced pressure system produces oxygen and/or hydroxyl radicals and oxidizes using these species (in-situ-steam-generation, ISSG). Further, although this exemplary embodiment refers to use of a reactor chamber, the method of the present invention is equally applicable to rapid thermal process (RTP) furnaces, vertical and horizontal tube furnaces, and other oxidation tools known in the industry.
In a specific exemplary embodiment, the substrate 113 is a silicon wafer. In this embodiment, the silicon wafer is placed onto the platen 11 and the process chamber 109 is pumped down to a subatmospheric pressure, of approximately 3 Torr to 20 Torr, or in certain applications, to a range of about 5 Torr to 6 torr. In other embodiments, ranges extending from 3 Torr to 300 Torr are contemplated. The oxygen butterfly valve 103 is initially closed and the hydrogen butterfly valve 101 is open. Although particular gas flow rates are not critical, particular oxygen flow rates that work in a specific chamber type are from 5 to 15 liters/minute with a hydrogen glow rate of 1% to 33% of the oxygen flow rate. The hydrogen gas enters the process chamber 109 and flows over the face of the silicon wafer. The hydrogen reduction process, as with most chemical reactions, becomes more efficient at elevated temperatures. In this embodiment, temperatures in a range of 750° C. to 1150° C. are employed. The wafer may either be heated by the lamp-type heaters 107 (e.g., tungsten-halogen lamps in light pipes) or through the substrate platen 111 (e.g., a resistive heating element—not shown). Hydrogen removes hydroxyls from a surface of the silicon wafer and reduces any elemental or compound metallic atoms or molecules as well as reduces any organic and inorganic contaminants. The hydrogen gas is typically left in the process chamber 109 for anywhere from 5 to 300 seconds, after which the process chamber 109 is evacuated through the pump 115. (A skilled artisan will recognize that the pump may be a series of pumps, such as a roughing pump and a turbomolecular pump although such details are not critical for application of the present invention.) After the process chamber 109 is evacuated, the silicon wafer 113 is oxidized. To oxidize the silicon wafer 113, the hydrogen butterfly valve 101 is closed and the oxygen butterfly valve 103 is opened (although any of the oxidation techniques described herein may be readily employed). Notice that the silicon wafer 113 has not been disturbed and remains in the process chamber 109, thereby preventing formation of any native oxide. Oxygen is allowed to flow as needed until a silicon dioxide film (not shown) formed on the silicon wafer is of a desired thickness.
The process flow chart 200 of
In the foregoing specification, the present invention has been described with reference to specific embodiments thereof. It will, however, be evident to a skilled artisan that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. For example, skilled artisans will appreciate the methods described herein are not exclusive and may be supplemented by other cleaning methodologies and techniques. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.
